The public switched telephone network's (PSTN) primary service has been to provide Plain Old Telephone Service (POTS), which has been accomplished using circuit switching technology and techniques. Traditionally, calls were established using circuits having defined bandwidth and characteristics suitable for voice transfer. In contrast, the Internet is a packet switched network designed for data transfer in which bandwidth can be allocated as needed. The PSTN and the Internet are evolving, to an extent, so as to adopt certain aspects and capabilities of the other.
One service that is involved in the evolution of both networks is voice traffic. At a high level, the traditional circuit switch telephone network is migrating towards transporting voice using packet switching technology. This offers the prospect of increased efficiencies and utilization of common equipment for voice, data, and video traffic. Similarly, the Internet that was initially developed to handle data transfer between computers has been adapted to carry voice calls. Various approaches have been created to convey voice over the Internet (“VoIP”), and many of the capabilities present in the PSTN are being adapted to VoIP environments
While the public networks and the Internet are evolving, so are private networks. Private networks are typically based on customer premises equipment (CPE) owned and operated by businesses. Private networks allow deployment of features that are not readily available from the public network providers often at a lower cost. The switching equipment is typically in the form of a private branch exchange (PBX) switch. Larger businesses (e.g., an “enterprise”) may deploy multiple PBXs at different locations and interconnect them using tie-lines or tie-trunks. This allows calls to be routed within the private network to remote locations. The PBXs typically have interfaces to the public switched telephone network (PSTN) for handling calls to users “off” the private network.
Enterprises deploying PBXs have also discovered the benefits of integrating voice on IP based networks. However, doing so can complicate the interworking between the PSTN, IP networks, and other PBXs on the private networks. Typically, PBXs simply maintain their existing arrangement of interfaces and add a new interface for voice over IP networks. While this can offer the potential of saving money on certain types of calls, the overall complexity increases. Therefore, it is desired that a common architecture and network service be defined to accommodate the migration of PBXs to an IP network for various services.